Ion chromatography (IC) is a widely used analytical technique for the determination of anionic and cationic analytes in various sample matrices. Typical separation columns for IC have an internal diameter ranging from about 2 to 4 millimeters and are operated at flow rates ranging from 0.2 to 3 mL/min. In an effort to improve the performance of IC, research has been performed to develop separation columns with smaller diameters. Such smaller columns are typically referred to as a capillary separation column when the internal diameter is about 1 millimeter or less.
The use of capillary separation columns has been found to improve the separation efficiency and/or reduce the analysis time. In addition to improved separation performance, capillary IC offers numerous other advantages such as a small sample volume requirement (e.g., 1 microliter or less) and a lower eluent usage. A capillary ion chromatography system typically operates at a flow rate ranging from 1 to 20 microliters/minute, and thus, the amount of eluent consumed is very small. The relatively low eluent usage makes long term continuous operation economically reasonable. For example, the total eluent usage for an entire year may only be about 5 liters using a typical capillary flow rate. A continuously operated always “on” system reduces the number of problems and time associated with system start-up and shutdown. In addition, low flow rates improve the system compatibility with a mass spectrometer. Another advantage of low volume capillary columns is that the required amount of stationary phase resin is relatively low. In turn, this either reduces costs or makes it economically feasible to use more expensive and exotic materials in the column.
In capillary IC, the eluent can be pumped to the capillary column using an isocratic or a concentration gradient eluent delivery mode. In ion chromatography, the eluent typically is an aqueous solution containing acid, or base, or salts. The eluent may also contain some organic solvents. The chemical composition of eluent delivery mode can be selected based on the chemical properties of the analyte, matrix, and capillary column. In the isocratic mode, the composition of the eluent is constant for the duration of the sample analysis. In contrast, the concentration gradient mode provides an eluent composition that changes as a function of time during the sample analysis. For example, the eluent composition can have one or more components such as, for example, an acid, base, or salt where the component concentration changes as a function of time.
In conventional scale IC that uses a 2 to 4 millimeter (internal diameter) separation column, concentration gradient eluent delivery with two or more components is typically performed with a high-pressure pump fitted with a low-pressure proportioning valve. Such a pump can typically be capable of proportioning up to four different eluent components that can each be independently varied as a function of time. The conventional scale concentration gradient pumps can have delay volumes of about 1 mL associated with the pump head and proportioning valve. This delay volume is not compatible with capillary scale IC separations since it corresponds to a delay time of about 100 minutes if the separation flow rate is about 10 microliters/minute. The delay time to change the eluent composition can be about the same or exceed the assay time of a typical capillary IC analysis. Clearly, the delay time must be substantially less than the assay time to have an effective concentration gradient eluent delivery. As such, Applicants believe that there is a need for concentration gradient eluent delivery systems having lower dead volumes than those observed with conventional scale concentration gradient pumps. Applicants also believe that concentration gradient eluent delivery systems with relatively low dead volume will enhance the separation capabilities of capillary chromatography such as, for example, in the analysis of carbohydrates and amino acids, where aqueous multi-component eluents at varying concentrations are typically required.